In U.S. Pat. No. 3,378,929 there is described a precision friction wheel distance measuring device which is particularly adapted for use in combination with machine tools where the device is used to measure the distance one part of the tool is moved relative to another part. For example, the measuring device may be mounted on a lathe carriage with a motion sensing wheel of the device engaging a guideway surface of the lathe bed to measure the distance the carriage is moved along the bed. Originally the sensing wheel was formed with a cylindrical surface. Subsequently it was discovered that measurement errors were encountered using such devices due to a phenomenon known as "metal elastic crowding" which varies in magnitude depending upon the difference in elastic properties of the sensing wheel and the metal defining the surface (the "measurement surface") track along which the wheel rolls. The wheel is made of a metal which is generally harder than the metal defining the measurement surface. Since this phenomenon, metal elastic crowding, tends to make the wheel appear to have a greater diameter than actually measured on a static basis, it was found the reading of the device could be corrected by changing the effective diameter of the wheel, as by giving the peripheral surface of the wheel a spherical contour and adjusting the tilt angle of the axis of rotation to change the effective radius of the wheel. By mounting the device so that the plane of rotation of the sensing wheel can be tilted out of exact perpendicularity to the measurement surface, the indicated measurement by the device can be adjusted by an amount adequate to correct for the effects of metal elastic crowding. However, the mounting which provides the tilting adjustment required to calibrate the instrument complicates the installation and increases the cost.
While tilting of the spherically contoured wheel permits adjustment to provide a virtually exact measurement accuracy, the repeatability of the device, namely, the ability of the device to read "zero" when returned to its original position after traversing the normal length of its travel, still remained a problem. In U.S. Pat. No. 3,771,228 there is described an arrangement for improving the repeatability of measurements by forming the sensing wheel with a plurality of ridges extending parallel to the axis of rotation and which are randomly spaced about the periphery of the wheel. Such ridges are formed, for example, by grinding the surface of the sensing wheel in a direction transverse to the circumference of the wheel.
The sensing wheel of these devices originally was formed with a smooth peripheral surface, whether or not the surface was spherically contoured. The wheel, in use, was forcefully engaged with the measurement surface and rolled faithfully along the surface due to friction. Hence, the term "friction" has been used in the art to describe the general type of measuring device to which this invention pertains. The wheel has also been referred to in the art as a "metering" wheel because, as it rolls along the measurement surface in use of the device, it meters the distance traversed by the device along the measurement surface. However, with the advent of fine striations in the peripheral surface of the sensing wheel in directions other than purely circumferentially of the wheel, it is seen that the term "friction", in the strictest sense of the word, is somewhat of a misnomer. Nevertheless, the term "friction" is used herein with reference to the sensing wheel because of the widespread acceptance of this term as descriptive of this type of measuring device.